Conventional optical position-measuring devices for recording the relative position of two movable objects usually include a scanning unit, as well as a scale movable to it in at least one measuring direction. The scanning unit and scale are in each case connected to the two objects, e.g., to machine parts movable relative to each other. In general, the correct mounting of the scanning unit and scale relative to each other proves to be problematic. An unwanted twisting of scale and scanning unit about an axis perpendicular to the scale is usually referred to as a “moiré twisting”. The results of such moiré twistings in measuring operation are a decline of the scanning-signal intensity and unwelcome errors in the position measurement.
For example, it is possible to reduce moiré errors using an optical position-measuring device as described in PCT International Published Patent Application No. WO 02/23131. In addition to a scale, e.g., taking the form of a linear reflected-light scale, it includes a scanning unit displaceable relative thereto in at least one measuring direction. In addition to a scanning grating several optoelectronic detector elements, inter alia, at least one optical reflector element in the form of a retro-reflector is disposed on the side of the scanning unit. The partial beams of rays, reflected for the first time by the scale, are reflected via the retro-reflector back in the direction of the scale. There, the partial beams of rays are subsequently reflected a second time before interfering partial beams of rays ultimately arrive at the detector elements and there, generate scanning signals modulated as a function of displacement. In the position-measuring device, the reflector element takes the form of a roof prism having optical retro-reflector functionality. Further position-measuring devices having optical reflector elements in the form of prism-like retro-reflector elements are described, for instance, in European Published Patent Application No. 0 387 520 and European Published Patent Application No. 1 347 271.